Bleaching cold caustic pump with so2



United States Patent 3,055,792 BLEACHING COLD CAUSTTC PUMP WITH S0 Andreas von Koeppen and Manfred W. Liithgens, Covington, Va., assignors to West Virginia Pulp and Paper Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 12, 1958, Ser. No. 754,538 4 Claims. (Cl. 162-25) Our present invention relates to the method of bleaching so-called cold caustic pulp (a hardwood pulp) with S0 In the manufacture of newsprint and similar inexpensive publication papers, a common practice is to grind logs of softwoods, especially spruce, to produce so-called groundwood with yields as high as 96%. When it is attempted to grind hardwoods in a similar manner, the pulps produced are generally unsatisfactory. It has been found however, that the grinding process as applied to hardwoods will offer good prospect of success if the Wood is first chipped in the usual manner and then the chips treated with a cold caustic solution followed by either fiberizing in disk refiners or a combination of screw pressing and disk refining to produce a high yield of 85 to 90%. The pulp produced however, lacks brightness to a considerable degree, and for use in white publication papers, requires bleaching in more or less conventional manner. However, to subject the pulp to conventional bleaching methods is expensive in the items of handling and consumption of chemicals, and furthermore, results in a substantially lower yield. It is therefore the object of the present invention to devise a cheap and satisfactory method of bleaching this type of pulp with only slight loss of the pulp substance.

Our invention is based upon our discovery that surprisingly, the cold caustic pulp may be bleached to the extent of approximately points on the Hunter scale by the simple application of S0 with only slight diminution of the yield. The following flow diagram illustrates the basic practice of our invention:

HARDWOOD CHIPS GOLD CAUSTIC PULPIN G MECHANICAL F IBEHIZATION COLD CAUST IC SOLUT ION V S0 BLEACH CHELATING AGENT 7 FOR IRON (OPTIONAL) BLEACHED HARDWOOD PULP Our invention will be best understood by reference to the following examples, and the features of novelty will be pointed out in the appended claims.

Example 1 3,055,792 Patented Sept. 25, 1962 quantity of this pulp was placed in a tumbling digester or autoclave at 10% consistency at C. and 3% of S0 applied for 30 minutes. The so-treated pulp had a Hunter brightness of 61.6% with a yield of 99.7%. (In other runs somewhat lower yields were had, e.g., 97.3%.) The so-treated pulp was then water washed and preferably additioned by 0.1 gram per liter of sodium tripolyphosphate or other chelating agent, for the purpose of inhibiting the adverse effect of dissolved iron upon the pulp color.

Example 2 This example was conducted similar-1y to Example 1, except that the pulp, instead of having been prepared from fresh wood, was prepared from Wood that was six months old, and fairly dry. The brightness was improved from 38.3 to 46.3%.

Example 3 Example 1 was repeated with mixed oak which was dry, and had been cut for a period of 6 months. The brightness improved from 45 to 53.8%.

It will be noted that the fresher the Wood, the greater the improvement of the brightness. The S0 may be applied either in the form of gas, in which case the S0 dissolves in the water of the pulp, or S0 water can be added as such, i.e., as sulfurous acid H 80 Also the bleaching solution can be made up from calcium base sulfite cooking liquor by adding sufficient S0 so that, of the total S0 present, not more than 20% is combined with the calcium. The presence of the calcium seems to offer some advantage in preserving yield, especially when the bleaching temperature is above 87 0., although below this temperature, no advantage is had by the presence of calcium. Sodium base cooking liquor may be similarly availed of, provided that sufficient free S 0 is present to bring the pH of the pulp undergoing bleaching to a pH of 3 or below, with best results at a pH of 2. This pH of 2 is the optimum for the bleaching, regardless of the exact composition of the bleach liquor. At least 1% of S0 should be used on the wood of the OD pulp. The main reaction takes place in the first 5 minutes of contact. After the first 5 minutes, only about 2 points increase will be had. After the first 30 minutes, very little improvement is noted. The consistency of the pulp undergoing bleaching is not critical, and consistencies between 1 and 15% are recommended. Above 15 there is less opportunity for contact of the fibers with the bleach. The preferred temperature for carrying out the bleaching lies in the range of 80 C. to C. The reaction is considerably slower below 80 C. and at about 50 C. and below, the degree of brightness obtained is substantially reduced. Above 110 C. there is a reduction of opacity and a loss in yield.

In order to insure against the adverse effect of slight quantities of iron in the mill waters, especially those used for washing the bleached pulp, the presence of a chelating agent, such as sodium tripolyphosphate, the Versenes, hexametaphosphate, etc. is recommended.

While in practice one stage of bleaching is recommended, the bleaching may be carried out in two or more stages, if desired.

The action of S0 on the cold caustic hardwood pulp is to be distinguished from its action on softwood groundwood, its action on the latter being much diminished. In general, softwood groundwood cannot be bleached more than 5 or 6 points in this manner, and its acquired brightness is quite temporary.

Various changes may be made in the steps specifically set forth, without departing from the spirit of our invention or the scope of the appended claims.

We claim:

1. The method of bleaching cold caustic hardwood pulp obtained in a yield of between 85 and 90% by treating chipped hardwood with caustic solution followed by mechanical fiberizing, which consists in applying to said pulp from 1 to 3% S0 based on the OD pulp, for a period of at least five minutes but less than the time required to reduce the pulp yield to less than 90% of the cold caustic pulp started with, at a temperature between 50 and 110 C. and at a pH between 2 and 3, and thereafter water washing the pulp.

2. The method of claim 1 wherein the S0 is applied to a water slurry of pulp having a consistency between 1 and 15%.

3. The method of claim 1 wherein a substantial portion but less than 20% of the S0 employed is in the form of a calcium salt and the temperature is above about 87 C.

4. The method of claim 1 wherein a chelating agent for iron is added to the pulp after water washing.

References Cited in the file of this patent UNITED STATES PATENTS 1,631,834 Schorger June 7, 1927 2,041,745 Dreyfus May 26, 1936 2,069,943 Dreyfus v Feb. 9, 1937 2,474,034 Craig June 21, 1949 2,707,144 Sparrow Apr. 26, 1955 2,707,145 Sparrow Apr. 26, 1955 

1. THE METHOD OF BLEACHING COLD CAUSTIC HARDWOOD PULP OBTAINED IN A YIELD OF BETWEEN 85 AND 90% BY TREATING CHAINED HARDWOOD WITH CAUSTIC SOLUTION FOLLOWED BY MECHANICAL FIBERIZING, WHICH CONSISTS IN APPLYING TO SAID PULP FROM 1 TO 3% SO2, BASED ON THE OD PULP, FOR A PERIOD OF AT LEAST FIVE MINUTES BUT LESS THAN THE TIME REQUIRED TO REDUCE THE PULP YIELD TO LESS THAN 90% OF THE COLD CAUSTIC PULP STARTED WITH, AT A TEMPERATURE BETWEEN 50 AND 110*C. AND AT A PH BETWEEN 2 AND 3, AND THEREAFTER WATER WASHING THE PULP. 